/*Problem Id:2601  User Id:Latex 
Memory:80K  Time:15MS
Language:C++  Result:Accepted

Source 
*/
#include <stdio.h>
#include <string.h>
#define maxn 3003

double sumc[maxn], c[maxn];
int n;

main()
{
	int i,j,k;
	double sum=0, d, a0, an1, a1;
	scanf("%d", &n);
	scanf("%lf%lf", &a0, &an1);
	sumc[0]=0;
	for (i=1;i<=n;i++)
	{
		scanf("%lf", c+i);
		sumc[i]=sumc[i-1]+c[i];
	}
	for (i=1;i<=n;i++)
		sum+=sumc[i];
	a1=(an1-a0-2*sum)/(n+1)+a0;
	printf("%.2lf\n", a1);
}

